do you mean 21:9 aspect ratio, like some gaming monitors? you can't change the aspect ratio of Platform Builder. it's stuck to 16:9.
you can simulate the effect by adding making a letterbox (black bars on top and bottom).
here is the easiest way to do it. place "area boundaries" tiles like in the pictures. it looks OK as long as you don't move the camera, so I recommend adding orange "View Boundaries".
![](https://i.ibb.co/1GhnmCW/Screenshot-png2.png)
![](https://i.ibb.co/dQJZ4VX/Screenshot.png)
there is a better solution, but only if you own the Pro DLC. let me know if that's the case. otherwise, I hope this was helpful
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